U.S. Pat. No. 4,384,925 discloses a sensing apparatus wherein a sensor housing having a sensor is mounted in a flow line for an air sample to be investigated. From time to time, the flow line is interrupted upstream and downstream of the sensor so that a definite quantity of the air sample is enclosed in this line segment and the sensor housing. The sensor operates pursuant to the coulometric principle so that the measured charge carriers during a time period predetermined by a control unit are a measure for the gas concentration contained in the closed-off air-sample chamber. This value is then utilized as a calibration value for the continuous measurement which follows by means of a second independent sensor.
However, the calibration sensor in the above-mentioned sensing apparatus is continuously subjected to the air sample also before the actual calibration so that when the calibration begins with the closing off of the sensor housing, a non-reproducible gas concentration is present which often changes. These unstable measurement conditions can considerably delay the measurement time until stable conditions are reached. Furthermore, the interrupted segments of the flow line are likewise to be viewed as portions of the measurement chamber wherein voids of the air sample having a concentration other than in the sensor chamber can become trapped and which contribute to falsifying the measuring result.
A further sensing device is disclosed in U.S. Pat. No. 3,854,320. This sensing device includes a sample chamber in which the gas to be investigated is introduced via an inlet and is released to the ambient via an open outlet after the sample chamber has been flushed. In this embodiment, the gas to be investigated is the exhaled breathing gas of a subject person. A sensor is disposed in the interior of the sample chamber and is covered with a shield or cover during the flushing phase. The sensor senses a component of gas contained in the air sample and for the embodiment shown, the gas component is alcohol contained in the breath. If a significant air sample is present in the sample chamber after a sufficiently long flushing duration, the cover over the sensor is lifted so that the latter is freely exposed to the air sample to be investigated in the sample chamber. During this exposure time, the sensor delivers a signal to an evaluation unit which then indicates the concentration of the gas component to be sensed in the sample chamber. The inlet is closed during the measuring time so that no further air sample can flow into the sample chamber. The cover is again placed over the sensor when the measuring step is to be ended so that the sensor is sealed with respect to the sample chamber. At the same time, the sensor and the space enclosed by the sensor cover are flushed with ambient air to remove any residual quantity of the sample gas which is still present in the encapsulated sensor chamber.
In this known sensing device, the sample chamber is separated only from the inlet with respect to flow during the measurement. However, a continuous and significan gas exchange with the ambient atmosphere takes place via the open outlet so that the initial concentration of the gas to be sensed during the measurement continuously reduces because of the external influences.